It is well known the effectiveness of oil-type drawing lubricants is primarily a function of viscosity. For high viscosity lubricants, extremely thin films e.g. 50 microinch (0.001 mm) are adequate to permit deep drawing of a metal sheet. One such lubricant used primarily on ferrous metal sheet for automotive applications includes a dispersion of a high viscosity liquid polar lubricant dispersed as a discontinuous phase in an anhydrous liquid having a relatively low viscosity. This two-phase lubricant is disclosed in U.S. Pat. No. 4,374,168, incorporated herein by reference.
Liquid lubricants are generally applied to a metal sheet by direct impingement onto a metal sheet or onto an applicator roller. The lubricated metal sheet must pass between high pressure steel or synthetic squeegee rollers to insure the lubricant is completely distributed over the entire surface of the metal sheet. However, it is difficult to obtain high viscosity thin lubricant films having a thickness less than 1000 microinch (0.025 mm). This excess lubricant causes severe problems if applied to coiled metal sheet in a steel mill. Because of the high viscosity of the thick film, it is difficult to apply sufficient tension to tightly rewind a coil during a processing operation. Such a coil tends to "squat" when removed from a rewinding mandrel. If the coil inner diameter closes excessively, the coil must be scraped since it cannot be unwound in subsequent processing. Such a loosely wound coil also tends to telescope during handling which can damage the coil and creates a safety hazard. If the sheet is stacked in cut lengths, feeding to a blanking press is difficult because the excess film thickness makes it difficult to break the surface tension between sheets.
More recently, there have been various attempts to apply a thin film of lubricant having a uniform thickness to a metal sheet by atomizing the lubricant into a spray of small droplets or mist. When the spray nozzles creating the mist are directed substantially perpendicular toward the metal sheet, the lubricant tends to be deposited in bands along the longitudinal direction of the sheet. This results in a non-uniform film thickness along the transverse direction even when using a squeegee roller. Others have enclosed the coating area with a spray chamber. A mist is sprayed into the chamber where the lubricant particles are charged electrostatically. The charged particles are attracted to the surface of the metal sheet. This approach is also undesirable because of the maintenance problems associated with the electrical equipment and recycling excess lubricant condensing within the chamber.
Accordingly, there remains a long felt need for a lubricated metal sheet having a thin film of uniform thickness and that is economical to produce. More particularly, there is a need to coat a thin film of high viscosity lubricant having a uniform thickness onto a metal sheet.